1. Field of the Invention
This invention relates to an actuation circuit of a piezoelectric transformer which converts a voltage of a dc voltage source into a predetermined voltage using a piezoelectric effect and an actuating method of a piezoelectric transformer.
2. Description of the Related Arts
A piezoelectric transformer is a voltage converting device wherein an ac voltage is applied to primary electrodes to generate mechanical oscillations through a piezoelectric effect so that to obtain a converted stable radio voltage from secondary electrodes, and is used for an invertor or a like device which generates a high voltage for a cold cathode fluorescent lamp.
An actuating circuit of the piezoelectric transformer is disclosed in Japanese Patent Application Laid-Open No. 47265/1996 (hereinafter referred to as conventional example 1). As shown in FIG. 1, the piezoelectric transformer converts current flowing to load 5 connected to piezoelectric transformer 1 into a voltage using resistor 38. This signal is supplied to amplifier 37 through phase shifter circuit 36 to actuate flip-flop 33.
Two transistor switching circuits 34 and 35 are actuated by Q and .sub.Q , output signals having opposite phase frequency divided by flip-flop 33 so that two pulse voltages of a resonance frequency of piezoelectric transformer 1 having phases different by 180 degrees from each other are generated from dc power supply voltage Vdd. The two pulse voltages are applied to primary side electrodes 1a and 1b of piezoelectric transformer 1 so that a voltage obtained by conversion by piezoelectric transformer 1 is extracted from secondary side electrode 1c. In other words, an conventional example 1 shown in FIG. 1, piezoelectric transformer 1 is actuated by a rectangular wave.
As another method of actuating piezoelectric transformer 1, the prior art of Japanese Patent Application Laid-Open No. 33349/1996 (hereinafter referred to as conventional example 2) is disclosed. According to the method, as shown in FIG. 2, electromagnetic transformer 41 is provided in a stage preceding to the input side of piezoelectric transformer 1, and a primary voltage of electromagnetic transformer 41 is detected by voltage detecting circuit 40 and switching transistor 42 is switched with a frequency corresponding to a value of the voltage. Thus, making use of resonance provided by the inductance on the secondary side of electromagnetic transformer 41 and an equivalent input capacitance of piezoelectric transformer 1, a half-sine wave having a voltage boosted from power supply voltage Vdd of FIG. 2 is generated.
An equivalent circuit of a piezoelectric transformer is generally represented as in FIG. 12 and is set such that it resonates with a resonance frequency of piezoelectric transformer 1 principally provided by equivalent input capacitance Cdl 27 and an inductance on the secondary side of electromagnetic transformer 41 in the preceding stage. The method described above is characterized in that piezoelectric transformer 1 is actuated with a half sine wave and a boosted voltage is outputted to load 5.
As additional prior art, a method wherein piezoelectric transformer 1 is actuated with a full-wave sine wave as in Japanese Patent Application Laid-open No. 27553/1996 (hereinafter referred to as conventional example 3) is disclosed. This actuating circuit is constructed, as shown in FIG. 3, the secondary sides of two autotransformers 43 and 44 are connected to two primary electrodes of piezoelectric transformer 1 while the primary sides of auto-transformers 43 and 44 are connected to power supply voltage Vdd. Further, switching transistors 47 and 48 are connected to centertaps of auto-transformers 43 and 44, and current flowing to load 5 is detected and an actuating frequency for frequency piezoelectric transformer 1 is determined based on the detected current by frequency control circuit 45 and inputted to 2-phase actuating circuit 46. Switching transistors 47 and 48 are alternately switched with the resonance frequency of piezoelectric transformer 1 in accordance with outputs of 2-phase actuating circuit 46.
The actuating circuit of FIG. 3 is set such that a resonance circuit is formed from the inductances of the primary sides and the secondary sides of switching transistors 47 and 48 and principally equivalent input capacitance Cdl 27 of the equivalent circuit of piezoelectric transformer 1 shown in FIG. 12. Thus, half sine waves of two phases illustrated in FIG. 3(A) and 3(B) are generated, and the half sine waves of two phases are applied alternately to the two primary side electrodes of piezoelectric transformer 1. This corresponds to the fact that a sine wave is applied to the primary side electrodes of piezoelectric transformer 1. Piezoelectric transformer 1 can apply a boosted output voltage to load 5 in this manner.
As described above, as the methods of actuating piezoelectric transformer 1, three actuating methods such as the method of inputting the rectangular waves, the method of inputting the half sine waves and the method of inputting full sine waves are known in the prior art.
The first problem resides in that, when a piezoelectric transformer is actuated with a waveform other than the sine waves such as the rectangular waves or a half sine wave, the actuating efficiency of piezoelectric transformer 1 is deteriorated. The reason is that, when the piezoelectric transformer is actuated, for example, with the rectangular waves as in Japanese Patent Application Laid-Open No. 47265/1996 (conventional example 1), since the rectangular wave includes harmonic components of the resonance frequency of the piezoelectric transformer, piezoelectric transformer 1 is oscillated also by the harmonic components. For example, if a rectangular wave of FIG. 4(A) is Fourier expanded to calculate a Fourier series f(t), it can be represented as expression (1): EQU f(t)=(4E)/.pi.{sin .theta.+(1/3)sin 3.theta.+(1/5)sin .theta.+ EQU (1/7)sin 7.theta.+. . . }. (1)
From expression (1), it can be seen that the rectangular wave includes a sine wave of a fundamental frequency and additional sine waves of frequencies equal to odd-numbered times the fundamental frequency. By representing this expression (1) in a graph separately for the different frequency terms, a waveform diagram shown in FIG. 4(B) is obtained. Accordingly, when piezoelectric transformer 1 is actuated with the rectangular wave, also the sine waves having frequencies equal to odd-numbered times the resonance frequency are inputted to piezoelectric transformer 1, and piezoelectric transformer 1 is oscillated also with the harmonic components.
Meanwhile, since energy of piezoelectric transformer 1 other than that of frequencies in the proximity of the resonance frequency cannot be extracted as output power, the oscillations of harmonic components of the resonance frequency become a loss. Accordingly, when the piezoelectric transformer is actuated using a rectangular wave, the efficiency is deteriorated. On the other hand, when the piezoelectric transformer is actuated with a half sine wave as in the conventional example 2, harmonic components of the resonance frequency are inputted to piezoelectric transformer 1 similarly. By calculating a Fourier series of a half sine wave shown in FIG. 4(C), it can be represented as expression (2): EQU f(t)=1/.pi.+(cos .theta.)/2+(2/.pi.{((1/3)cos 2.theta.- EQU (1/15)cos 4.theta.+. . . }. (2)
Also the expression (2) includes harmonic components, and it is apparent that the actuating efficiency is deteriorated compared with that where a full sine wave is used. From the foregoing explanation, in order to efficiently actuate piezoelectric transformer 1, it is desirable to use a full sine wave.
The second problem resides in that, when half sine waves of two phases are generated in order to achieve efficient actuation as in the actuating circuit of the conventional example 3, two electromagnetic transformers are required, resulting in drawbacks that a high cost is required and that also the invertor has an increased physical size.
The reason is that, if piezoelectric transformer 1 is actuated with a sine wave without making use of a fly-back voltage of an electromagnetic transformer or a coil, power loss occurs in the actuating circuit. Where piezoelectric transformer 1 is actuated with a sine wave without using a fly-back voltage, an amplifier of the class A must be used as an actuating stage. However, when an intermediate voltage between a power supply voltage and the GND potential like a sine wave is outputted, a voltage corresponding to a difference between the power supply voltage and the voltage applied to piezoelectric transformer 1 is applied to the output transistor, and power equal to the product of the voltage and current flowing out from the output transistor is consumed by the transistor.
Since this power becomes a loss of the actuating circuit, where a piezoelectric transistor is actuated with a sine wave using an amplifier of the class A, the actuating circuit exhibits a loss, resulting in a problem that the efficiency of the entire invertor is deteriorated. Since half sine waves of two phases must be produced using two electromagnetic transformers, a high cost and a large physical size are required.
The third problem is that the input voltage range of the piezoelectric transformer actuating circuit is narrow. The reason is that the actuating voltage inputted to piezoelectric transformer 1 is, in the conventional example 1, a rectangular wave having an amplitude equal to twice that of power supply voltage Vdd from the GND potential, and also the actuating voltage of piezoelectric transformer 1 increases in proportion to the power supply voltage. Also in the conventional example 2 or 3, since a power supply voltage is applied to the primary side of an electromagnetic transformer so that a fly-back voltage is produced on the primary side and a boosted voltage is extracted from the secondary side of the electromagnetic transformer to actuate piezoelectric transformer 1, the actuating voltage of piezoelectric transformer 1 increases in proportion to power supply voltage Vdd. Therefore, the three conventional examples 1 to 3 actuate to feed a fixed output current and voltage to load 5 by varying a boosting ratio raising the actuating frequency of piezoelectric transformer 1.
Generally, since piezoelectric transformer 1 operates in the highest efficiency with the resonance frequency thereof and the efficiency drops as the frequency thereto is displaced away from the resonance frequency, there is a limitation to absorb the variation of the power supply voltage. For example, if the power supply voltage increases to twice, then piezoelectric transformer 1 must be operated with a half boosting ratio. Therefore, where the equivalent input capacitance of piezoelectric transformer 1 and the inductance of the electromagnetic transformer are set so as to cause resonance, as in conventional example 2 or example 3, since the actuating waveform does not vary, the switching period can be varied but in a very small range. Consequently, it is generally difficult to obtain a twice or more input voltage range.
Further, as the power supply voltage rises, the loss of the actuating circuit increases. Where the power supply voltage is represented by Vdd, the primary side inductance of the electromagnetic transformer is represented by L1 and the time during when the switching transistor is on is represented by t, current Id flowing to the primary side of the electromagnetic transformer and the switching transistor is represented by expression (3): EQU Id=Vdd.times.t/L1 (3)
Accordingly, from expression (3), the current flowing to the electromagnetic transformer and the switching transistor increases in proportion to the power supply voltage, and as the power supply voltage of the piezoelectric transformer actuating circuit increases, the current flowing to the electromagnetic transformer and the switching transistor increases in proportion to the power supply voltage. However, since the output voltage of piezoelectric transformer 1 is controlled to a fixed level, surplus power inputted to the electromagnetic transformer is returned to the power supply side. Consequently, the loss by a series resistance component of the electromagnetic transformer and the switching transistor increases, and also the efficiency of the piezoelectric transformer actuating circuit is deteriorated. As described above, the conventional examples have a drawback that the input voltage range is narrow.